Control systems for a treadmill including a control unit and different sensors are available.
U.S. Pat. No. 5,314,391 describes an ultrasonic distance meter which continuously measures the distance from the body of a user and adapts the speed of the treadmill accordingly. The ultrasonic sensor allows a distance measurement at a resolution of several centimeters.
U.S. Pat. No. 5,368,532 describes an automatic treadmill speed control system including two pressure sensors beneath the running surface for detecting the position of a user. U.S. Pat. Nos. 7,094,180 B2 and 7,101,319 describe similar approaches. These systems, however, require the use of a plurality of sensors beneath the treadmill to measure the position of the foot with sufficient accuracy and reliability. They are not able to directly measure the position of the body of the user.
U.S. Pat. No. 6,135,924 describes an automatic treadmill control system including an optical position sensor, an infrared sensor, and a calibration system.
U.S. Pat. No. 6,126,575 describes a treadmill control system including a self-retracting rope which is attached to the user. A sensor detects the retraction or extension of the rope and adjusts the treadmill speed accordingly to the running speed of the user.
A safety key or a safety clip, which is attached to the user with the aid of a safety rope and a clamp, is most common. If the user suddenly moves backward on the treadmill and the safety clip detaches, the treadmill is deactivated or an emergency braking is carried out. Such an approach is described in the China Patent No. CN202277640, for example.